Process for eliminating oxides of nitrogen

Abstract

The invention concerns materials for eliminating oxides of nitrogen NO and NO2 present in exhaust gases, in particular from the internal combustion engines of automotive vehicles operating in a medium which is super-stoichiometric in oxidising agents, which can adsorb oxides of nitrogen then desorb the oxides of nitrogen by elevating the temperature with respect to the adsorption temperature or by passage of rich mixture, said materials comprising mixed oxides the metals of which are in octahedral coordination, with the octahedra connecting together so that the structure generates micropores in the form of channels. These materials adsorb oxides of nitrogen by insertion and do not become poisoned in contact with oxides of sulphur and carbon contained in the gases. In the presence of a group VIII metal, these materials are capable of eliminating oxides of nitrogen absorded by reduction during passage of a rich mixture.

What is claimed is:  
     
       1. A process for eliminating oxides of nitrogen from exhaust gases, from automotive vehicle internal combustion engines, comprising contacting the exhaust gases with a material selected from materials with a cryptomelane, hollandite, romanechite or todorkite structure, comprising MO 6  octahedra connected together such that the structure generates micropores in the form of channels, said octahedra comprising at least one element M selected from the group consisting of elements from groups IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB and IIIA of the periodic table, said octahedra excluding tin, said material further comprising at least one element (B) selected from the group consisting of alkali-metal elements, alkaline-earth elements, rare earth elements, transition metals and elements from groups IIIA, IVA of the periodic table. 
     
     
       2. A process according to  claim 1 , wherein the average valency of the metals (M) in the octahedra is about +4. 
     
     
       3. A process according to  claim 1 , wherein said at least one element (M) is manganese, tungsten, zirconium, titanium, molybdenum or chromium. 
     
     
       4. A process according to  claim 3 , said material further comprising at least one element M selected from the group consisting of aluminum, zinc, copper, nickel, cobalt, iron, chromium, scandium and yttrium. 
     
     
       5. A process according to  claim 1 , said material further comprising at least one noble metal (C) from group VIII of the periodic table. 
     
     
       6. A process according to  claim 5 , wherein said element (C) is platinum. 
     
     
       7. A process according to  claim 1 , wherein said material comprises, as a percentage by weight: 
       40% to 80% of at least one metal M;  
       0.01% to 30% of at least one element (B);  
       optionally, 0.05% to 5% of at least one noble metal (C) from group VIII of the periodic table.  
     
     
       8. A process according to  claim 1 , wherein said material has a specific surface in the range of 20 to 300 m 2 /g. 
     
     
       9. A process according to  claim 1 , wherein said material comprises at least one porous support. 
     
     
       10. A process according to  claim 9 , characterized in that the porous support is selected from the group consisting of: SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , SiC, MgO and alumino-silicate. 
     
     
       11. A process according to  claim 1 , wherein in that said material comprises at least one rigid support. 
     
     
       12. A process according to  claim 1 , wherein adsorption of the oxides of nitrogen is carried out at a temperature in the range of 50° C. to 400° C. 
     
     
       13. A process according to  claim 12 , said process further comprising a step for desorbing the oxides of nitrogen. 
     
     
       14. A process according to  claim 13 , wherein thermal desorption of the oxides of nitrogen is carried out at a temperature in the range of 300° C. to 500° C. 
     
     
       15. A process according to  claim 12 , said process further comprising a step for desorbing the oxides of nitrogen implemented by varying the gas composition. 
     
     
       16. A process according to  claim 15 , wherein chemical desorption of the oxides of nitrogen is carried out at a temperature in the range of 150° C. to 500° C. 
     
     
       17. A process according to  claim 13 , further comprising a step for reducing the oxides of nitrogen to molecular nitrogen and/or nitric oxide. 
     
     
       18. A process according to  claim 17 , wherein reduction of the oxides of nitrogen takes place in the presence of a catalyst comprising at least one inorganic refractory oxide, optionally at least one zeolite, at least one element selected from the group consisting of elements from transition metal groups VIB, VIIB, VIII and IIB, optionally at least one noble metals of group VIII, and optionally at least one element selected from the group of elements from the alkaline-earth group IIA, and the rare earth group IIB. 
     
     
       19. A process according to  claim 17 , wherein the step for adsorbing the oxides of nitrogen, the step for desorbing the oxides of nitrogen and the step for reducing the oxides of nitrogen take place in the presence of said material, the latter further comprising a noble metal from group VIII of the periodic table. 
     
     
       20. A process according to  claim 1 , wherein the internal combustion engine is a diesel engine. 
     
     
       21. A process for eliminating oxides of nitrogen from exhaust gases, from automotive vehicle internal combustion engines, comprising contacting the exhaust gases with a material selected from materials with a cryptomelane, hollandite, romanechite or todorkite structure, comprising MO 6  octahedra connected together such that the structure generates micropores in the form of channels, said octahedra comprising at least one element M selected from the group consisting of elements from groups IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIIB and IIIA of the periodic table, said material further comprising at least one element (B) selected from the group consisting of alkali-metal elements, alkaline-earth elements, rare earth elements, transition metals and elements from groups IIIA, IVA of the periodic table, said material further comprising at least one noble metal (C) from group VIII of the periodic table. 
     
     
       22. A process for eliminating oxides of nitrogen from exhaust gases, from automotive vehicle internal combustion engines, comprising contacting the exhaust gases with a material selected from materials with a cryptomelane, hollandite, romanechite or todorkite structure, comprising MO 6  octahedra connected together such that the structure generates micropores in the form of channels, said octahedra comprising at least one element M selected from the group consisting of elements from groups IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB and IIIA of the periodic table, said material further comprising at least one element (B) selected from the group consisting of alkali-metal elements, alkaline-earth elements, rare earth elements, transition metals and elements from groups IIIA, IVA of the periodic table, said contacting comprising adsorbing oxides of nitrogen at a temperature in the range of 50° C. to 400° C., thermally and desorbing said oxides of nitrogen at a temperature in the range 300° C. to 500° C.°, further comprising a step for reducing the oxides of nitrogen to molecular nitrogen and/or nitric oxide, and wherein the step for adsorbing the oxides of nitrogen, the step for desorbing the oxides of nitrogen and the step for reducing the oxides of nitrogen take place in the contact with said material further comprising a noble metal from group VIII of the periodic table. 
     
     
       23. A process according to  claim 21 , wherein said element (C) is platinum. 
     
     
       24. A process according to  claim 1 , wherein said octahedral does not contain any element from group IVA of the periodic table.